1. Field of the Invention
This invention relates to a fuel supply control system for an internal combustion engine.
2. Description of the Prior Art
Some internal combustion engines are equipped with fuel injection valves via which fuel is injected into an air intake passage at a point downstream of a throttle valve. In most of these engines, and air flow meter senses the rate of air flow through the intake passage at a point upstream of the throttle valve. The rate of fuel injection is controlled basically in proportion to the sensed air flow rate to maintain a desired air-to-fuel ratio of the mixture entering the engine.
In the case where these engines have a turbocharger to boost the engine power output, the compressor of the turbocharger is generally disposed in the intake passage at a point upstream of the throttle valve but downstream of the air flow meter. In this case, when the throttle valve is rapidly closed to quickly reduce the engine speed from its cruising speed, the air-fuel mixture goes excessively rich as a result of the following considerations. Immediately after the closing of the throttle valve, the compressor continues to compress the air upstream of the throttle valve due to its inertia so that air continues to be driven through the air flow meter at a considerable rate. During this period, however, air flow is substantially obstructed by the throttle valve. Thus, the sensed air flow rate becomes considerably greater than the rate of actual air flow at the fuel injection valves downstream of the throttle valve. This greater apparent air flow rate results in an excessively high rate of fuel injection, since the rate of fuel injection is proportional to the sensed air flow rate.